Memory system

ABSTRACT

A memory system includes: a first memory device including a first memory and a first memory controller suitable for controlling the first memory to store data; a second memory device including a second memory and a second memory controller suitable for controlling the second memory to store data; and a processor suitable for executing an operating system (OS) and an application to access a data storage memory through the first and second memory devices.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional ApplicationNo. 62/242,377 filed on Oct. 16, 2015, which is incorporated herein byreference in its entirety.

BACKGROUND

1. Field

Various embodiments relate to a memory system and, more particularly, amemory system including plural heterogeneous memories having differentlatencies.

2. Description of the Related Art

In conventional computer systems, a system memory, a main memory, aprimary memory, or an executable memory is typically implemented by thedynamic random access memory (DRAM). The DRAM-based memory consumespower even when no memory read operation or memory write operation isperformed to the DRAM-based memory. This is because the DRAM-basedmemory should constantly recharge capacitors included therein. TheDRAM-based memory is volatile, and thus data stored in the DRAM-basedmemory is lost upon removal of the power.

Conventional computer systems typically include multiple levels ofcaches to improve performance thereof. A cache is a high speed memoryprovided between a processor and a system memory in the computer systemto perform an access operation to the system memory faster than thesystem memory itself in response to memory access requests provided fromthe processor. Such cache is typically implemented with a static randomaccess memory (SRAM). The most frequently accessed data and instructionsare stored within one of the levels of cache, thereby reducing thelumber of memory access transactions and improving performance.

Conventional mass storage devices, secondary storage devices or diskstorage devices typically include one or more of magnetic media (e.g.,hard disk drives), optical media (e.g., compact disc (CD) drive, digitalversatile disc (DVD), etc.), holographic media, and mass-storage flashmemory (e.g., solid state drives (SSDs) removable flash drives, etc.).These storage devices are Input/Output (I/O) devices because they areaccessed by the processor through various I/O adapters that implementvarious I/O protocols. Portable or mobile devices (e.g., laptops,netbooks, tablet computers, personal digital assistant (PDAs), portablemedia players, portable gaming devices, digital cameras, mobile phones,smartphones, feature phones, etc.) may include removable mass storagedevices (e.g., Embedded Multimedia Card (eMMC), Secure Digital (SD)card) that are typically coupled to the processor via low-powerinterconnects and I/O controllers.

A conventional computer system typically uses flash memory devicesallowed only to store data and not to change the stored data in order tostore persistent system information. For example, initial instructionssuch as the basic input and output system (BIOS) images executed by theprocessor to initialize key system components during the boot processare typically stored in the flash memory device. In order to speed upthe BIOS execution speed, conventional processors generally cache aportion of the BIOS code during the pre-extensible firmware interface(PEI) phase of the boot process.

Conventional computing systems and devices include the system memory orthe main memory, consisting of the DRAM, to store a subset of thecontents of system non-volatile disk storage. The main memory reduceslatency and increases bandwidth for the processor to store and retrievememory operands from the disk storage.

The DRAM packages such as the dual in-line memory modules (DIMMs) arelimited in terms of their memory density, and are also typicallyexpensive with respect to the non-volatile memory storage. Currently,the main memory requires multiple DIMMs to increase the storage capacitythereof, which increases the cost and volume of the system. Increasingthe volume of a system adversely affects the form factor of the system.For example, large DIMM memory ranks are not ideal in the mobile clientspace. What is needed is an efficient main memory system whereinincreasing capacity does not adversely affect the form factor of thehost system.

SUMMARY

Various embodiments of the present invention are directed to a memorysystem including plural heterogeneous memories having differentlatencies.

In accordance with an embodiment of the present invention, a memorysystem may include: a first memory device including a first memory and afirst memory controller suitable for controlling the first memory tostore data; a second memory device including a second memory and asecond memory controller suitable for controlling the second memory tostore data and a processor suitable for executing an operating system(OS) and an application to access a data storage memory through thefirst and second memory devices. The first and second memories may beseparated from the processor. The second memory controller may transfera signal between the processor and the second memory device based on atleast one of values of a handshaking information field included in thesignal. The first memory may include a high-capacity memory, which has alower latency than the second memory and operates as a cache memory forthe second memory, and a high-speed memory, which has a lower latencythan the high-capacity memory and operates as a cache memory for thehigh-capacity memory. The first memory controller may include ahigh-capacity memory cache controller suitable for controlling thehigh-capacity memory to store data, and a high-speed memory cachecontroller suitable for controlling the high-speed memory to store data.The second memory controller and the second memory may communicate witheach other through an input/output bus. The high-speed memory cachecontroller and the high-speed memory may communicate with each otherthrough a first input/output bus, which is a part of the input/outputbus, in a high-speed operation mode. The high-capacity memory cachecontroller and the high-capacity memory may communicate with each otherthrough a second input/output bus, which is another part of theinput/output bus, in a high-capacity operation mode. The high-capacitymemory may cache data of the second memory through the secondinput/output bus under a control of the second memory controller and thehigh-capacity memory cache controller in the high-speed operation mode.

In accordance with an embodiment of the present invention, a memorysystem may include: a first memory device including a first memory and afirst memory controller suitable for controlling the first memory tostore data; a second memory device including a second memory and asecond memory controller suitable for controlling the second memory tostore data; and a processor suitable for accessing the first and secondmemories. The second memory controller may transfer a signal between theprocessor and the second memory device based on at least one of valuesof a handshaking information field included in the signal. The firstmemory may include a high-capacity memory, which has a lower latencythan the second memory and operates as a cache memory for the secondmemory, and a high-speed memory, which has a lower latency than thehigh-capacity memory and operates as a cache memory for thehigh-capacity memory. The first memory controller may include ahigh-capacity memory cache controller suitable for controlling thehigh-capacity memory to store data, and a high-speed memory cachecontroller suitable for controlling the high-speed memory to store data.The second memory controller and the second memory may communicate witheach other through an input/output bus. The high-speed memory cachecontroller and the high-speed memory may communicate with each otherthrough a first input/output bus, which is a part of the input/outputbus, in a high-speed operation mode. The high-capacity memory cachecontroller and the high-capacity memory may communicate with each otherthrough a second input/output bus, which is another part of theinput/output bus, in a high-capacity operation mode. The high-capacitymemory may cache data of the second memory through the secondinput/output bus under a control of the second memory controller and thehigh-capacity memory cache controller in the high-speed operation mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating a structure ofcaches and a system memory according to an embodiment of the presentinvention.

FIG. 2 is a block diagram schematically illustrating a hierarchy ofcache—system memory—mass storage according to an embodiment of thepresent invention.

FIG. 3 is a block diagram illustrating a computer system according to anembodiment of the present invention.

FIG. 4 is a block diagram illustrating a two-level memory sub-systemaccording to an embodiment of the present invention.

FIG. 5 is a block diagram illustrating a two-level memory sub-systemaccording to an embodiment of the present invention.

FIG. 6 is a block diagram illustrating an input/output bus of thetwo-level memory sub-system of FIG. 5.

DETAILED DESCRIPTION

Various embodiments will be described below in more detail withreference to the accompanying drawings. The present invention may,however, be embodied in different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete andwill fully convey the scope of the present invention to those skilled inthe art. The drawings are not necessarily to scale and in someinstances, proportions may have been exaggerated to dearly illustratefeatures of the embodiments. Throughout the disclosure, referencenumerals correspond directly to like parts in the various figures andembodiments of the present invention. It is also noted that in thisspecification, “connected/coupled” refers to one component not onlydirectly coupling another component but also indirectly coupling anothercomponent through an intermediate component. In addition, a singularform may include a plural form as long as it is not specificallymentioned in a sentence. It should be readily understood that themeaning of “on” and “over” in the present disclosure should beinterpreted in the broadest manner such that “on” means not only“directly on” but also “on” something with an intermediate feature(s) ora layer(s) therebetween, and that “over” means not only directly on topbut also on top of something with an intermediate feature(s) or alayer(s) therebetween. When a first layer is referred to as being “on” asecond layer or “on” a substrate, it not only refers to a case in whichthe first layer is formed directly on the second layer or the substratebut also a case in which a third layer exists between the first layerand the second layer or the substrate.

FIG. 1 is a block diagram schematically illustrating a structure ofcaches and a system memory according to an embodiment of the presentinvention.

FIG. 2 is a block diagram schematically illustrating a hierarchy ofcache—system memory—mass storage according to an embodiment of thepresent invention.

Referring to FIG. 1 the caches and the system memory may include aprocessor cache 110, an internal memory cache 131, an external memorycache 135 and a system memory 151. The internal and external memorycaches 131 and 135 may be implemented with a first memory 130 (see FIG.3), and the system memory 151 may be implemented with one or more of thefirst memory 130 and a second memory 150 (see FIG. 3).

For example, the first memory 130 may be volatile and may be the DRAM.

For example, the second memory 150 may be non-volatile and may be one ormore of the NAND flash memory, the NOR flash memory and a non-volatilerandom access memory (NVRAM). Even though the second memory 150 may beexemplarily implemented with the NVRAM, the second memory 150 will notbe limited to a particular type of memory device.

The NVRAM may include one or more of the ferroelectric random accessmemory (FRAM) using a ferroelectric capacitor, the magnetic randomaccess memory (MRAM) using the tunneling magneto-resistive (TMR) layer,the phase change random access memory (PRAM) using a chalcogenide alloy,the resistive random access memory (RERAM) using a transition metaloxide, the spin transfer torque random access memory (STT-RAM), and thelike.

Unlike a volatile memory, the NVRAM may maintain its content despiteremoval of the power. The NVRAM may also consume less power than a DRAM.The NVRAM may be of random access. The NVRAM may be accessed at a lowerlevel of granularity (e.g., byte level) than the flash memory. The NVRAMmay be coupled to a processor 170 over a bus, and may be accessed at alevel of granularity small enough to support operation of the NVRAM asthe system memory (e.g., cache line size such as 64 or 128 bytes). Forexample, the bus between the NVRAM and the processor 170 may be atransactional memory bus (e.g., a DDR bus such as DDR3, DDR4, etc.). Asanother example, the bus between the NVRAM and the processor 170 may bea transactional bus including one or more of the PCI express (PCIE) busand the desktop management interface (DMI) bus, or any other type oftransactional bus of a small-enough transaction payload size (e.g.,cache line size such as 64 or 128 bytes). The NVRAM may have fasteraccess speed than other non-volatile memories, may be directly writablerather than requiring erasing before writing data, and may be morere-writable than the flash memory.

The level of granularity at which the NVRAM is accessed may depend on aparticular memory controller and a particular bus to which the NVRAM iscoupled. For example, in some implementations where the NVRAM works as asystem memory, the NVRAM may be accessed at the granularity of a cacheline (e.g., a 64-byte or 128-Byte cache line), at which a memorysub-system including the internal and external memory caches 131 and 135and the system memory 151 accesses a memory. Thus, when the NVRAM isdeployed as the system memory 151 within the memory sub-system, theNVRAM may be accessed at the same level of granularity as the firstmemory 130 (e.g., the DRAM) included in the same memory sub-system. Evenso, the level of granularity of access to the NVRAM by the memorycontroller and memory bus or other type of bus is smaller than that ofthe block size used by the flash memory and the access size of the I/Osubsystem's controller and bus.

The NVRAM may be subject to the wear leveling operation due to the factthat storage cells thereof begin to wear out after a number of writeoperations. Since high cycle count blocks are most likely to wear outfaster, the wear leveling operation may swap addresses between the highcycle count blocks and the low cycle count blocks to level out memorycell utilization. Most address swapping may be transparent toapplication programs because the swapping is handled by one or more ofhardware and lower-level software (e.g., a low level driver or operatingsystem).

The phase-change memory (PCM) or the phase change random access memory(PRAM or PCRAM) as an example of the NVRAM is a non-volatile memoryusing the chalcogenide glass. As a result of heat produced by thepassage of an electric current, the chalcogenide glass can be switchedbetween a crystalline state and an amorphous state. Recently the PRAMmay have two additional distinct states. The PRAM may provide higherperformance than the flash memory because a memory element of the PRAMcan be switched more quickly, the write operation changing individualbits to either “1” or “0” can be done without the need to firstly erasean entire block of cells, and degradation caused by the write operationis slower. The PRAM device may survive approximately 100 million writecycles.

For example, the second memory 150 may be different from the SRAM, whichmay be employed for dedicated processor caches 113 respectivelydedicated to the processor cores 111 and for a processor common cache115 shared by the processor cores 111; the DRAM configured as one ormore of the internal memory cache 131 internal to the processor 170(e.g., on the same die as the processor 170) and the external memorycache 135 external to the processor 170 (e.g., in the same or adifferent package from the processor 170); the flash memory/magneticdisk/optical disc applied as the mass storage (not shown); and a memory(not shown) such as the flash memory or other read only memory (ROM)working as a firmware memory, which can refer to boot ROM and BIOSFlash.

The second memory 150 may work as instruction and data storage that isaddressable by the processor 170 either directly or via the first memory130. The second memory 150 may also keep pace with the processor 170 atleast to a sufficient extent in contrast to a mass storage 251B. Thesecond memory 150 may be placed on the memory bus, and may communicatedirectly with a memory controller and the processor 170.

The second memory 150 may be combined with other instruction and datastorage technologies (e.g., DRAM) to form hybrid memories, such as, forexample, the Co-locating PRAM and DRAM, the first level memory and thesecond level memory, and the FLAM (i.e., flash and DRAM).

At least a part of the second memory 150 may work as mass storageinstead of, or in addition to, the system memory 151. When the secondmemory 150 serves as a mass storage 251A, the second memory 150 servingas the mass storage 251A need not be random accessible, byte addressableor directly addressable by the processor 170.

The first memory 130 may be an intermediate level of memory that haslower access latency relative to the second memory 150 and/or moresymmetric access latency (i.e., having read operation times which areroughly equivalent to write operation times). For example, the firstmemory 130 may be a volatile memory such as volatile random accessmemory (VRAM) and may comprise the DRAM or other high speedcapacitor-based memory. However, the underlying principles of theinvention will not be limited to these specific memory types. The firstmemory 130 may have a relatively lower density. The first memory 130 maybe more expensive to manufacture than the second memory 150.

In one embodiment, the first memory 130 may be provided between thesecond memory 150 and the processor cache 110. For example, the firstmemory 130 may be configured as one or snore external memory caches 135to mask the performance and/or usage limitations of the second memory150 including, for example, read/write latency limitations and memorydegradation limitations. The combination of the external memory cache135 and the second memory 150 as the system memory 151 may operate at aperformance level which approximates, is equivalent or exceeds a systemwhich uses only the DRAM as the system memory 151.

The first memory 130 as the internal memory cache 131 may be located onthe same die as the processor 170. The first memory 130 as the externalmemory cache 135 may be located external to the die of the processor170. For example, the first memory 130 as the external memory cache 135may be located on a separate die located on a CPU package, or located ona separate die outside the CPU package with a high bandwidth link to theCPU package. For example, the first memory 130 as the external memorycache 135 may be located on a dual in-line memory module (DIMM), ariser/mezzanine, or a computer motherboard. The first memory 130 may becoupled in communication with the processor 170 through a single ormultiple high bandwidth links, such as the DDR or other transactionalhigh bandwidth links.

FIG. 1 illustrates how various levels of caches 113, 115, 131 and 135may be configured with respect to a system physical address (SPA) spacein a system according to an embodiment of the present invention. Asillustrated in FIG. 1, the processor 170 may include one or moreprocessor cores 111, with each core having its own internal memory cache131. Also, the processor 170 may include the processor common cache 115shared by the processor cores 111. The operation of these various cachelevels are well understood in the relevant art and will not be describedin detail here.

For example, one of the external memory caches 135 may correspond to oneof the system memories 151, and serve as the cache for the correspondingsystem memory 151. For example, some of the external memory caches 135may correspond to one of the system memories 151, and serve as thecaches for the corresponding system memory 151. In some embodiments, thecaches 113, 115 and 131 provided within the processor 170 may performcaching operations for the entire SPA space.

The system memory 151 may be visible to and/or directly addressable bysoftware executed on the processor 170. The cache memories 113, 115, 131and 135 may operate transparently to, the software in the sense thatthey do not form a directly-addressable portion of the SPA space whilethe processor cores 111 may support execution of instructions to allowsoftware to provide some control (configuration, policies, hints, etc.)to some or all of the cache memories 113, 115, 131 and 135.

The subdivision into the plural system memories 151 may be performedmanually as part of a system configuration process (e.g., by a systemdesigner) and/or may be performed automatically by software.

In one embodiment, the system memory 151 may be implemented with one ormore of the non-volatile memory (e.g., PRAM) used as the second memory150, and the volatile memory (e.g., DRAM) used as the first memory 130.The system memory 151 implemented with the volatile memory may bedirectly addressable by the processor 170 without the first memory 130serving as the memory caches 131 and 135.

FIG. 2 illustrates the hierarchy of cache—system memory—mass storage bythe first and second memories 130 and 150 and various possible operationmodes for the first and second memories 130 and 150.

The hierarchy of cache—system memory—mass storage may comprise a cachelevel 210, a system memory level 230 and a mass storage level 250, andadditionally comprise a firmware memory level (not illustrated).

The cache level 210 may include the dedicated processor caches 113 andthe processor common cache 115, which are the processor cache.Additionally, when the first memory 130 serves in a cache mode for thesecond memory 150 working as the system memory 151B, the cache level 210may further include the internal memory cache 131 and the externalmemory cache 135.

The system memory level 230 may include the system memory 151Bimplemented with the second memory 150. Additionally, when the firstmemory 130 serves in a system memory mode, the system memory level 230may further include the first memory 133 working as the system memory151A.

The mass storage level 250 may include one or more of theflash/magnetic/optical mass storage 251B and the mass storage 215Aimplemented with the second memory 150.

Further, the firmware memory level may include the BIOS flash (notillustrated) and the BIOS memory implemented with the second memory 150.

The first memory 130 may serve as the caches 131 and 135 for the secondmemory 150 working as the system memory 151B in the cache mode. Further,the first memory 130 may serve as the system memory 151A and occupy aportion of the SPA space in the system memory mode.

The first memory 130 may be partitionable, wherein each partition mayindependently operate in a different one of the cache mode and thesystem memory mode. Each partition may alternately operate between thecache mode and the system memory mode. The partitions and thecorresponding modes may be supported by one or more of hardware,firmware, and software. For example, sizes of the partitions and thecorresponding modes may be supported by a set of programmable rangeregisters capable of identifying each partition and each mode within amemory cache controller 270.

When the first memory 130 serves in the cache mode for the system memory151B, the SPA space may be allocated not to the first memory 130 workingas the memory caches 131 and 135 but to the second memory 150 working asthe system memory 151B. When the first memory 130 serves in the systemmemory mode, the SPA space may be allocated to the first memory 130working as the system memory 151A and the second memory 150 working asthe system memory 151B.

When the first memory 130 serves in the cache mode for the system memory151B, the first memory 130 working as the memory caches 131 and 135 mayoperate in various sub-modes under the control of the memory cachecontroller 270. In each of the sub-modes, a memory space of the firstmemory 130 may be transparent to software in the sense that the firstmemory 130 does not form a directly-addressable portion of the SPAspace. When the first memory 130 serves in the cache mode, the sub-modesmay include but may not be limited as of the following table 1.

TABLE 1 MODE READ OPERATION WRITE OPERATION Write-Back Allocate on CacheMiss Allocate on Cache Miss Cache Write-Back on Evict of Write-Back onEvict of Dirty Data Dirty Data 1^(st) Memory Bypass to 2^(nd) MemoryBypass to 2^(nd) Memory Bypass 1^(st) Memory Allocate on Cache MissBypass to 2^(nd) Memory Read-Cache & Cache Line InvalidationWrite-Bypass 1^(st) Memory Allocate on Cache Miss Update Only on CacheRead-Cache & Hit Write-Through to Write-Through 2^(nd) Memory

During the write-back cache mode, part of the first memory 130 may workas the caches 131 and 135 for the second memory 150 working as thesystem memory 151B. During the write-back cache mode, every writeoperation is directed initially to the first memory 130 working as thememory caches 131 and 135 when a cache line, to which the writeoperation is directed, is present in the caches 131 and 135. Acorresponding write operation is performed to update the second memory150 working as the system memory 151B only when the cache line withinthe first memory 130 working as the memory caches 131 and 135 is to bereplaced by another cache line.

During the first memory bypass mode, all read and write operationsbypass the first memory 130 working as the memory caches 131 and 135 andare performed directly to the second memory 150 working as the systemmemory 151B. For example, the first memory bypass mode may be activatedwhen an application is not cache-friendly or requires data to beprocessed at the granularity of a cache line. In one embodiment, theprocessor caches 113 and 115 and the first memory 130 working as thememory caches 131 and 135 may perform the caching operationindependently from each other. Consequently, the first memory 130working as the memory caches 131 and 135 may cache data, which is notcached or required not to be cached in the processor caches 113 and 115,and vice versa. Thus, certain data required not to be cached in theprocessor caches 113 and 115 may be cached within the first memory 130working as the memory caches 131 and 135.

During the first memory read-cache and rite-bypass mode, a read cachingoperation to data from the second memory 150 working as the systemmemory 151B may be allowed. The data of the second memory 150 working asthe system memory 151B may be cached in the first memory 130 working asthe memory caches 131 and 135 for read-only operations. The first memoryread-cache and write-bypass mode may be useful in the case that mostdata of the second memory 150 working as the system memory 151B is “readonly” and the application usage is cache-friendly.

The first memory read-cache and write-through mode may be considered asa variation of the first memory read-cache and write-bypass mode. Duringthe first memory read-cache and write-through mode, the write-hit mayalso be cached as well as the read caching. Every write operation to thefirst memory 130 working as the memory caches 131 and 135 may cause awrite operation to the second memory 150 working as the system memory151B. Thus, due to the write-through nature of the cache, cache-linepersistence may be still guaranteed.

When the first memory 130 works as the system memory 151A, all or partsof the first memory 130 working as the system memory 151A may bedirectly visible to an application and may form part of the SPA space.The first memory 130 working as the system memory 151A may be completelyunder the control of the application. Such scheme may create thenon-uniform memory address (NUMA) memory domain where an applicationgets higher performance from the first memory 130 working as the systemmemory 151A relative to the second memory 150 working as the systemmemory 151B. For example, the first memory 130 working as the systemmemory 151A may be used for the high performance computing (HPC) andgraphics applications which require very fast access to certain datastructures.

In an alternative embodiment, the system memory mode of the first memory130 may be implemented by pinning certain cache lines in the firstmemory 130 working as the system memory 151A, wherein the cache lineshave data also concurrently stored in the second memory 150 working asthe system memory 151B.

Although not illustrated, parts of the second memory 50 may be used asthe firmware memory. For example, the parts of the second memory 150 maybe used to store BIOS images instead of or in addition to storing theBIOS information in the BIOS flash. In this case, the parts of thesecond memory 150 working as the firmware memory may be a part of theSPA space and may be directly addressable by an application executed onthe processor cores 111 while the BIOS flash may be addressable throughan I/O sub-system 320.

To sum up, the second memory 150 may serve as one or more of the massstorage 215A and the system memory 151B. When the second memory 150serves as the system memory 151B and the first memory 130 serves as thesystem memory 151A, the second memory 150 working as the system memory151B may be coupled directly to the processor caches 113 and 115. Whenthe second memory 150 serves as the system memory 151B but the firstmemory 130 serves as the cache memories 131 and 135, the second memory150 working as the system memory 151B may be coupled to the processorcaches 113 and 115 through the first memory 130 working as the memorycaches 131 and 135. Also, the second memory 150 may serve as thefirmware memory for storing the BIOS images.

FIG. 3 is a block diagram illustrating a computer system 300 accordingto an embodiment of the present invention.

The computer system 300 may include the processor 170 and a memory andstorage sub-system 330.

The memory and storage sub-system 330 may include the first memory 130,the second memory 150, and the flash/magnetic/optical mass storage 251B.The first memory 130 may include one or more of the cache memories 131and 135 working in the cache mode and the system memory 151A working inthe system memory mode. The second memory 150 may include the systemmemory 151B, and may further include the mass storage 251A as an option.

In one embodiment, the NVRAM may be adopted to configure the secondmemory 150 including the system memory 151B, the mass storage 251A forthe computer system 300 for storing data, instructions, states, andother persistent and non-persistent information.

Referring to FIG. 3, the second memory 150 may be partitioned into thesystem memory 151B and the mass storage 251A, and additionally thefirmware memory as an option.

For example, the first memory 130 working as the memory caches 131 and135 may operate as follows during the write-back cache mode.

The memory cache controller 270 may perform the look-up operation inorder to determine whether the read-requested data is cached in thefirst memory 130 working as the memory caches 131 and 135.

When the read-requested data is cached in the first memory 130 workingas the memory caches 131 and 135, the memory cache controller 270 mayreturn the read-requested data from the first memory 130 working as thememory caches 131 and 135 to a read requestor (e.g., the processor cores111).

When the read-requested data is not cached in the first memory 130working as the memory caches 131 and 135, the memory cache controller270 may provide a second memory controller 311 with the data readrequest and a system memory address. The second memory controller 311may use a decode table 313 to translate the system memory address to aphysical device address (PDA) of the second memory 150 working as thesystem memory 151B, and may direct the read operation to thecorresponding region of the second memory 150 working as the systemmemory 151B. In one embodiment, the decode table 313 may be used for thesecond memory controller 311 to translate the system memory address tothe PDA of the second memory 150 working as the system memory 151B, andmay be updated as part of the wear leveling operation to the secondmemory 150 working as the system memory 151B. Alternatively, a part ofthe decode table 313 may be stored within the second memory controller311.

Upon receiving the requested data from the second memory 150 working asthe system memory 151B, the second memory controller 311 may return therequested data to the memory cache controller 270, the memory cachecontroller 270 may store the returned data in the first memory 130working as the memory caches 131 and 135 and may also provide thereturned data to the read requestor. Subsequent requests for thereturned data may be handled directly from the first memory 130 workingas the memory caches 131 and 135 until the returned data is replaced byanother data provided from the second memory 150 working as the systemmemory 151B.

During the write-back cache mode when the first memory 130 works as thememory caches 131 and 135, the memory cache controller 270 may performthe look-up operation in order to determine whether the write-requesteddata is cached in the first memory 130 working as the memory caches 131and 135. During the write-back cache mode, the write-requested data maynot be provided directly to the second memory 150 working as the systemmemory 151B. For example, the previously write-requested and currentlycached data may be provided to the second memory 150 working as thesystem memory 151B only when the location of the previouslywrite-requested data currently cached in first memory 130 working as thememory caches 131 and 135 should be re-used for caching another datacorresponding to a different system memory address. In this case, thememory cache controller 270 may determine that the previouslywrite-requested data currently cached in the first memory 130 working asthe memory caches 131 and 135 is currently not in the second memory 150working as the system memory 151B, and thus may retrieve the currentlycached data from first memory 130 working as the memory caches 131 and135 and provide the retrieved data to the second memory controller 311.The second memory controller 311 may look up the PDA of the secondmemory 150 working as the system memory 151B for the system memoryaddress, and then may store the retrieved data into the second memory150 working as the system memory 151B.

The coupling relationship among the second memory controller 311 and thefirst and second memories 130 and 150 of FIG. 3 may not necessarilyindicate particular physical bus or particular communication channel. Insome embodiments, a common memory bus or other type of bus may be usedto communicatively couple the second memory controller 311 to the secondmemory 150. For example, in one embodiment, the coupling relationshipbetween the second memory controller 311 and the second memory 150 ofFIG. 3 may represent the DDR-typed bus, over which the second memorycontroller 311 communicates with the second memory 150. The secondmemory controller 311 may also communicate with the second memory 150over a bus supporting a native transactional protocol such as the PCIEbus, the DMI bus, or any other type of bus utilizing a transactionalprotocol and a small-enough transaction payload size (e.g., cache linesize such as 64 or 128 bytes).

In one embodiment, the computer system 300 may include an integratedmemory controller 310 suitable for performing a central memory accesscontrol for the processor 170. The integrated memory controller 310 mayinclude the memory cache controller 270 suitable for performing a memoryaccess control to the first memory 130 working as the memory caches 131and 135, and the second memory controller 311 suitable for performing amemory access control to the second memory 150.

In the illustrated embodiment, the memory cache controller 270 mayinclude a set of mode setting information which specifies variousoperation mode (e.g., the write-back cache mode, the first memory bypassmode, etc.) of the first memory 130 working as the memory caches 131 and135 for the second memory 150 working as the system memory 151B. Inresponse to a memory access request, the memory cache controller 270 maydetermine whether the memory access request may be handled from thefirst memory 130 working as the memory caches 131 and 135 or whether thememory access request is to be provided to the second memory controller311, which may then handle the memory access request from the secondmemory 150 working as the system memory 151B.

In an embodiment where the second memory 150 is implemented with PRAM,the second memory controller 311 may be a PRAM controller. Despite thatthe PRAM is inherently capable of being accessed at the granularity ofbytes, the second memory controller 311 may access the PRAM-based secondmemory 150 at a lower level of granularity such as a cache line (e.g., a64-bit or 128-bit cache line) or any other level of granularityconsistent with the memory sub-system. When PRAM-based second memory 150is used to form a part of the SPA space, the level of granularity may behigher than that traditionally used for other non-volatile storagetechnologies such as the flash memory, which may only perform therewrite and erase operations at the level of a block (e.g., 64 Kbytes insize for the NOR flash memory and 16 Kbytes for the NAND flash memory).

In the illustrated embodiment the second memory controller 311 may readconfiguration data from the decode table 313 in order to establish theabove described partitioning and modes for the second memory 150. Forexample, the computer system 300 may program the decode table 313 topartition the second memory 150 into the system memory 151B and the massstorage 251A. An access means may access different partitions of thesecond memory 150 through the decode table 313. For example, an addressrange of each partition is defined in the decode table 333.

In one embodiment, when the integrated memory controller 310 receives anaccess request, a target address of the access request may be decoded todetermine whether the request is directed toward the system memory 151B,the mass storage 251A, or I/O devices.

When the access request is a memory access request, the memory cachecontroller 270 may further determine from the target address whether thememory access request is directed to the first memory 130 working as thememory caches 131 and 135 or to the second memory 150 working as thesystem memory 151B. For the access to the second memory 150 working asthe system memory 151B, the memory access request may be forwarded tothe second memory controller 311.

The integrated memory controller 310 may pass the access request to theI/O sub-system 320 when the access request is directed to the I/Odevice. The I/O sub-system 320 may further decode the target address todetermine whether the target address points to the mass storage 251A ofthe second memory 150, the firmware memory of the second memory 150, orother non-storage or storage I/O devices. When the further decodedaddress points to the mass storage 251A or the firmware memory of thesecond memory 150, the I/O sub-system 320 may forward the access requestto the second memory controller 311.

The second memory 150 may act a replacement or supplement for thetraditional DRAM technology in the system memory. In one embodiment, thesecond memory 150 working as the system memory 151B along with the firstmemory 130 working as the memory caches 131 and 135 may represent atwo-level system memory. For example, the two-level system memory mayinclude a first-level system memory comprising the first memory 130working the memory caches 131 and 135 and a second-level system memorycomprising the second memory 150 working as the system memory 151B.

According to some embodiments, the mass storage 251A implemented withthe second memory 150 may act as replacement or supplement for theflash/magnetic/optical mass storage 251B. In some embodiments, eventhough the second memory 150 is capable of byte-level addressability,the second memory controller 311 may still access the mass storage 251Aimplemented with the second memory 150 by units of blocks of multiplebytes (e.g., 64 Kbytes, 128 Kbytes, and so forth). The access to themass storage 251A implemented with the second memory 150 by the secondmemory controller 311 may be transparent to an application executed bythe processor 170. For example, even though the mass storage 251Aimplemented with the second memory 150 is accessed differently from theflash/magnetic/optical mass storage 251B, the operating system may stilltreat the mass storage 251A implemented with the second memory 150 as astandard mass storage device (e.g., a serial ATA hard drive or otherstandard form of mass storage device).

In an embodiment where the mass storage 251A implemented with the secondmemory 150 acts as replacement or supplement for theflash/magnetic/optical mass storage 251B, it may not be necessary to usestorage drivers for block-addressable storage access. The removal of thestorage driver overhead from the storage access may increase accessspeed and may save power. In alternative embodiments where the massstorage 251A implemented with the second memory 150 appears asblock-accessible to the OS and/or applications and indistinguishablefrom the flash/magnetic/optical mass storage 251B, block-accessibleinterfaces (e.g., Universal Serial Bus (USB), Serial Advanced TechnologyAttachment (SATA) and the like) may be exposed to the software throughemulated storage drivers in order to access the mass storage 251Aimplemented with the second memory 150.

In some embodiments, the processor 170 may include the integrated memorycontroller 310 comprising the memory cache controller 270 and the secondmemory controller 311, all of which may be provided on the same chip asthe processor 170, or on a separate chip and/or package connected to theprocessor 170.

In some embodiments, the processor 170 may include the I/O sub-system320 coupled to the integrated memory controller 310. The I/O sub-system320 may enable communication between processor 170 and one or more ofnetworks such as the local area network (LAN), the wide area network(WAN) or the internet; a storage I/O device such as theflash/magnetic/optical mass storage 251B and the BIOS flash; and one ormore of non-storage I/O devices such as display, keyboard, speaker, andthe like. The I/O sub-system 320 may be on the same chip as theprocessor 170, or on a separate chip and/or package connected to theprocessor 170.

The I/O sub-system 320 may translate a host communication protocolutilized within the processor 170 to a protocol compatible withparticular I/O devices.

In the particular embodiment of FIG. 3, the memory cache controller 270and the second memory controller 311 may be located on the same die orpackage as the processor 170. In other embodiments, one or more of thememory cache controller 270 and the second memory controller 311 may belocated off-die or off-package, and may be coupled to the processor 170or the package over a bus such as a memory bus such as the DDR bus, thePCIE bus, the DMI bus, or any other type of bus.

FIG. 4 is a block diagram illustrating a two-level memory sub-system 400according to an embodiment of the present invention.

Referring to FIG. 4, the two-level memory sub-system 400 may include thefirst memory 130 working as the memory caches 131 and 135 and the secondmemory 150 working as the system memory 151B. The two-level memorysub-system 400 may include a cached sub-set of the mass storage level250 including run-time data. In an embodiment, the first memory 130included in the two-level memory sub-system 400 may be volatile and theDRAM. In an embodiment, the second memory 150 included in the two-levelmemory sub-system 400 may be non-volatile and one or more of the NANDflash memory, the NOR flash memory and the NVRAM. Even though the secondmemory 150 may be exemplarily implemented with the NVRAM the secondmemory 150 will not be, limited to a particular memory technology.

The second memory 150 may be presented as the system memory 151B to ahost operating system (OS: not illustrated) while the first memory 130works as the caches 131 and 135, which is transparent to the OS, for thesecond memory 150 working as the system memory 151B. The two-levelmemory sub-system 400 may be managed by a combination of logic andmodules executed via the processor 170. In an embodiment, the firstmemory 130 may be coupled to the processor 170 through high bandwidthand low latency means for efficient processing. The second memory 150may be coupled to the processor 170 through low bandwidth and highlatency means.

The two-level memory sub-system 400 may provide the processor 170 withrun-time data storage. The two-level memory sub-system 400 may providethe processor 170 with access to the contents of the mass storage level250. The processor 170 may include the processor caches 113 and 115,which store a subset of the contents of the two-level memory sub-system400.

The first memory 130 may be managed by the memory cache controller 270while the second memory 150 may be managed by the second memorycontroller 311. In an embodiment, the memory cache controller 270 andthe second memory controller 311 may be located on the same, die orpackage as the processor 170. In other embodiments, one or more of thememory cache controller 270 and the second memory controller 311 may beorated off-die or off-package, and may be coupled to the processor 170or to the package over a bus such as a memory bus (e.g., the DDR bus)the PCIE bus, the DMI bus, or any other type of bus.

The second memory controller 311 may report the second memory 150 to thesystem OS as the system memory 151B. Therefore, the system OS mayrecognize the size of the second memory 150 as the size of the two-levelmemory sub-system 400. The system OS and system applications are unawareof the first memory 130 since the first memory 130 serves as thetransparent caches 131 and 135 for the second memory 150 working as thesystem memory 151B.

The processor 170 may further include a two-level management unit 410.The two-level management unit 410 may be a logical construct that maycomprise one or more of hardware and micro-code extensions to supportthe two-level memory sub-system 400. For example, the two-levelmanagement unit 410 may maintain a full tag table that tracks the statusof the second memory 150 working as the system memory 151B. For example,when the processor 170 attempts to access a specific data segment in thetwo-level memory sub-system 400, the two-level management unit 410 maydetermine whether the data segment cached in the first memory 130working as the caches 131 and 135. When the data segment is not cachedin the first memory 130, the two-level management unit 410 may fetch thedata segment from the second memory 150 working as the system memory151B and subsequently may write the fetched data segment to the firstmemory 130 working as the caches 131 and 135. Because the first memory130 works as the caches 131 and 135 for the second memory 150 working asthe system memory 151B, the two-level management unit 410 may furtherexecute data prefetching or similar cache efficiency processes known inthe art.

The two-level management unit 410 may manage the second memory 150working as the system memory 151B. For example, when the second memory150 comprises the non-volatile memory, the two-level management unit 410may perform various operations including wear-levelling, bad-blockavoidance, and the like in a manner transparent to the system software.

As an exemplified process of the two-level memory sub-system 400, inresponse to a request for a data operand, it may be determined whetherthe data operand is cached in first memory 130 working as the memorycaches 131 and 135. When the data operand is cached in first memory 130working as the memory caches 131 and 135, the operand may be returnedfrom the first memory 130 working as the memory caches 131 and 135 to arequestor of the data operand. When the data operand is not cached infirst memory 130 working as the memory caches 131 and 135, it may bedetermined whether the data operand is stored in the second memory 150working as the system memory 151B. When the data operand is stored inthe second memory 150 working as the system memory 151B, the dataoperand may be cached from the second memory 150 working as the systemmemory 151B into the first memory 130 working as the memory caches 131and 135 and then returned to the requestor of the data operand. When thedata operand is not stored in the second memory 150 working as thesystem memory 151B, the data operand may be retrieved from the massstorage 250, cached into the second memory 150 working as the systemmemory 151B, cached into the first memory 130 working as the memorycaches 131 and 135, and then returned to the requestor of the dataoperand.

FIG. 5 is a block diagram illustrating a two-level memory sub-system 500according to an embodiment of the present invention.

The two-level memory sub-system 500 of FIG. 5 may be the same as thetwo-level memory sub-system 400 of FIG. 4 except that the first memory130 working as the memory caches 131 and 135 may include a high-speedmemory 130A and a high-capacity memory 130B and that the memory cachecontroller 270 configured to control the first memory 130 may include ahigh-speed memory cache controller 270A configured to control thehigh-speed memory 130A and a high-capacity memory cache controller 270Bconfigured to control the high-capacity memory 130B.

The high-speed memory 130A may be a volatile memory suitable forhigh-speed memory operation, and may be the DRAM. The high-capacitymemory 130B may be a volatile memory suitable for caching a great amountof data, and may be the DRAM. The high-speed memory 130A may operatewith high bandwidth, very low latency, generally high cost and greatpower consumption. The high-capacity memory 130B may operate with highlatency, high caching capacity, low cost and small power consumptionwhen compared with the high-speed memory 130A. The high-capacity memory130B may operate with lower operation speed than the high-speed memory130A and with higher operation speed than the second memory 150. Thehigh-capacity memory 130B may have greater data storage capacity thanthe high-speed memory 130A, and smaller data storage capacity than thesecond memory 150. The high-speed memory 130A may serve as a cachememory for the high-capacity memory 130B, and the high-capacity memory130B may serve as a cache memory for the second memory 150.

The high-speed memory 130A and the high-capacity memory 130B may berespectively managed by the high-speed memory cache controller 270A andthe high-capacity memory cache controller 270B while the second memory150 may be managed by the second memory controller 311. In anembodiment, the high-speed memory cache controller 270A, thehigh-capacity memory cache controller 270B and the second memorycontroller 311 may be located on the same die or package as theprocessor 170. In other embodiments, one or more of the high-speedmemory cache controller 270A the high-capacity memory cache controller270B and the second memory controller 311 may be located off-die oroff-package, and may be coupled to the processor 170 or to the packageover a bus such as a memory bus (e.g., the DDR bus), the PIE bus, theDMI bus, or any other type of bus.

The system OS and system applications are unaware of the high-speedmemory 130A and the high-capacity memory 130B since the high-speedmemory 130A and the high-capacity memory 130B serve as the transparentcaches 131 and 135 for the second memory 150 working as the systemmemory 151B.

For example, when the processor 170 attempts to access a specific datasegment in the two-level memory sub-system 500, the two-level managementunit 410 may determine whether the data segment is cached in thehigh-speed memory 130A. When the data segment is not cached in thehigh-speed memory 130A, the two-level management unit 410 may determinewhether the data segment is cached in the high-capacity memory 130B.When the data segment is cached in the high-capacity memory 130B, thetwo-level management unit 410 may fetch the data segment from thehigh-capacity memory 130B and subsequently may write the fetched datasegment to the high-speed memory 130A. When the data segment is notcached in the high-capacity memory 130B, the two-level management unit410 may fetch the data segment from the second memory 150 working as thesystem memory 151B and subsequently may write the fetched data segmentto the high-speed memory 130A and the high-capacity memory 130B. Becausethe high-speed memory 130A and the high-capacity memory 130B work as thecaches 131 and 135 for the second memory 150 working as the systemmemory 151B, the two-level management unit 410 may further execute dataprefetching or similar cache efficiency processes known in the art.

As an example of a process of the two-level memory sub-system 500 ofFIG. 5, in response to a request for a data operand, it may bedetermined whether the data operand is cached in the high-speed memory130A working as the memory caches 131 and 135. When the data operand iscached in the high-speed memory 130A, the operand may be returned fromthe high-speed memory 130A to a requestor of the data operand.

When the data operand is not cached in the high-speed memory 130A, itmay be determined whether the data operand is stored in thehigh-capacity memory 130B working as the memory caches 131 and 135. Whenthe data operand is cached in the high-capacity memory 130B, the dataoperand may be cached from the high-capacity memory 130B into thehigh-speed memory 130A and then returned to the requestor of the dataoperand.

When the data operand is not cached in the high-capacity memory 130Bworking as the memory caches 131 and 135, it may be determined whetherthe data operand is stored in the second memory 150 working as thesystem memory 151B. When the data operand is stored in the second memory150, the data operand may be cached from the second memory 150 into thehigh-speed memory 130A and the high-capacity memory 130B working as thememory caches 131 and 135 and then returned to the requestor of the dataoperand.

When the data operand is not stored in the second memory 150, the dataoperand may be retrieved from the mass storage 250, cached into thesecond memory 150 working as the system memory 151B, cached into thehigh-speed memory 130A and the high-capacity memory 130B working as thememory caches 131 and 135, and then returned to the requestor of thedata operand.

The two-level memory sub-system 500 of FIG. 5 may further include acooling unit 510. The high-capacity memory 130B should periodicallyperform the refresh operation to a great number of memory cells, andtherefore the power consumption of the high-capacity memory 130B mayincrease due to the refresh operation. The cooling unit 510 may managethe temperature of the high-capacity memory 130B below a predeterminedvalue, which may increase the period of the refresh operation and thusprevent the increase of the power consumption of the high-capacitymemory 130B due to the refresh operation.

In accordance with an embodiment of the present invention, signalsexchanged between the processor 170 and a second memory unit, whichincludes the second memory controller 311 and the second memory 150working as the system memory 151B, may include a handshaking informationfield as well as a memory access request field and a correspondingresponse field (e.g., the read command, the write command, the address,the data, the data strobe, and so forth).

The handshaking information field may be for the second memory unit,which includes the second memory controller 311 and the second memory150 working as the system memory 151B, communicating with the processor170 through the handshaking scheme, and therefore may be included in thesignal exchanged between the processor 170 and the second memory unit,which includes the second memory controller 311 and the second memory150 working as the system memory 151B. For example, the handshakinginformation field may have three values according to types of the signalbetween the processor 170 and the second memory unit (the second memorycontroller 311 and the second memory 150 working as the system memory151B) as exemplified in the following table 2.

TABLE 2 HANDSHAKING FIELD SOURCE DESTINATION SIGNAL TYPE 10 PROCESSOR(170) 2^(ND) MEMORY DATA REQUEST UNIT (READ COMMAND) 11 2^(ND) MEMORYPROCESSOR (170) DATA READY UNIT 01 PROCESSOR (170) 2^(ND) MEMORY SESSIONSTART UNIT

As exemplified in table 2, the signals between the processor 170 and thesecond memory unit may include at least the data request signal (“DATAREQUEST (READ COMMAND)”) the data ready signal (“DATA READY”), and thesession start signal (“SESSION START”), which have binary values “10”,“11” and “01” of the handshaking information field, respectively.

The data request signal may be provided from the processor 170 to thesecond memory unit, and may indicate a request of data stored in thesecond memory unit. Therefore, for example, the data request signal mayinclude the read command and the read address as well as the handshakinginformation field having the value ‘10’ indicating the second memoryunit as the destination.

The data ready signal may be provided from the second memory unit to theprocessor 170 in response to the data request signal, and may have thehandshaking information field of the value “11” representingtransmission standby of the requested data, which is retrieved from thesecond memory unit in response to the read command and the read addressincluded in the data request signal.

The session start signal may be provided from the processor 170 to thesecond memory unit in response to the data ready signal, and may havethe handshaking information field of the value “01” representingreception start of the requested data ready to be transmitted in thesecond memory unit. For example, the processor 170 may receive therequested data from the second memory unit after providing the sessionstart signal to the second memory unit.

The processor 170 and the second memory controller 311 of the secondmemory unit may operate according to the signals between the processor170 and the second memory unit by identifying the type of the signalsbased on the value of the handshaking information field.

Although not illustrated, the second memory controller 311 may furtherinclude a handshaking interface unit. The handshaking interface unit mayreceive the data request signal provided from the processor 170, whichincludes the memory cache controller 270, and having the value “10” ofthe handshaking information field, and allow the second memorycontroller 311 to operate according to the data request signal. Also,the handshaking interface unit may provide the processor 170 with thedata ready signal having the value “01” of the handshaking informationfield in response to the data request signal from the processor 170.

As described above, the bus between the handshaking interface unit andthe processor 170 may be a transactional bus including one or more ofthe PCIE bus and the DMI bus, or any other type of transactional bus ofa small-enough transaction payload size (e.g., cache line size such as64 or 128 bytes). For example, when the second memory 150 works as thesystem memory 151B, the second memory 150 may be accessed at thegranularity of a cache line (e.g., a 64-byte or 128-Byte cache line), atwhich a memory sub-system including the first memory 130 working as theexternal memory caches 131 and 135 and the system memory 151 accesses amemory. Thus, when the second memory 150 is deployed as the systemmemory 151B within the memory sub-system, the second memory 150 may beaccessed at the same level of granularity as the first memory 130 (e.g.,the DRAM) included in the same memory sub-system. The couplingrelationship between the second memory controller 311 and the secondmemory 150 of FIGS. 4 and 5 may not necessarily indicate particularphysical bus or particular communication channel. In some embodiments, acommon memory bus or other type of bus may be used to communicativelycouple the second memory controller 311 to the second memory 150. Forexample, in an embodiment, the coupling relationship between the secondmemory controller 311 and the second memory 150 of FIGS. 4 and 5 mayrepresent a DDR-typed bus, over which the second memory controller 311communicates with the second memory 150. The second memory controller311 may also communicate with the second memory 150 over a bussupporting a native transactional protocol such as the PCIE bus, the DMIbus, or any other type of bus utilizing a transactional protocol and asmall-enough transaction payload size (e.g., cache line size such as 64or 128 bytes).

Although not illustrated, the second memory controller 311 may furtherinclude a register. The register may temporarily store the requesteddata retrieved from the second memory 150 working as the system memory151B in response to the data request signal from the processor 170. Thesecond memory controller 311 may temporarily store the requested dataretrieved from the second memory 150 working as the system memory 151Binto the register and then provide the processor 170 with the data readysignal having the value “01” of the handshaking information field inresponse to the data request signal.

As an exemplified process of the two-level memory sub-system 400 and 500of FIGS. 4 and 5, the processor 170 including the memory cachecontroller 270 may provide the second memory controller 311 with thedata request signal including the handshaking information field of thevalue “10” as well as the read command and the read address through thehandshaking interface unit. In response to the data request signal, thesecond memory controller 311 may read out requested data from the secondmemory 150 working as the system memory 151B according to the readcommand and the read address included in the data request signal. Thesecond memory controller 311 may temporarily store the read-out datainto the register. The second memory controller 311 may provide theprocessor 170 with the data ready signal through the handshakinginterface unit after the temporal storage of the read-out data into theregister. In response to the data ready signal, the processor 170 mayprovide the second memory controller 311 with the session start signalincluding the handshaking information field of the value “01”, and thenreceive the read-out data temporarily stored in the register.

As described above, in accordance with an embodiment of the presentinvention, the processor 170 may communicate with the second memory unitthrough the communication of the handshaking scheme and thus theprocessor 170 may perform another operation without stand-by untilreceiving requested data from the second memory unit.

When the processor 170 provides the second memory controller 311 withthe data request signal through the handshaking interface unit, theprocessor 170 may perform another data communication with another device(e.g., the I/O device coupled to the bus coupling the processor 170 andthe handshaking interface unit) until the second memory controller 311provides the processor 170 with the data ready signal. Further, uponreception of the data ready signal provided from the second memorycontroller 311, the processor 170 may receive the read-out datatemporarily stored in the register of the second memory controller 311by providing the session start signal to the second memory controller311 at any time the processor 170 requires the read-out data.

Therefore, in accordance with an embodiment of the present invention,the processor 170 may perform another operation without stand-by untilreceiving requested data from the second memory unit thereby improvingoperation bandwidth thereof.

FIG. 6 is a block diagram illustrating an input/output bus 610 of thetwo-level memory sub-system 500 of FIG. 5.

FIG. 6 illustrates the input/output bus 610 operatively coupling theprocessor 170 with the high-speed memory 130A, the high-capacity memory130B and the second memory 150 of the two-level memory sub-system 500.

Referring to FIG. 6, the input/output bus 610 may include a firstinput/output bus 620 and a second input/output bus 630 as parts thereof.

In accordance with an embodiment of the present invention, a high-speedoperation mode, where the high-speed memory 130A operates under thecontrol of the high-speed memory cache controller 270A, and ahigh-capacity operation mode, where the high-capacity memory 130Boperates under the control of the high-capacity memory cache controller270B, may be independently activated, or may be concurrently activatedwhen required. Therefore, the high-capacity memory 130B may be poweredoff during the high-speed operation mode only, while the high-speedmemory 130A may be powered off during the high-capacity operation modeonly. Such alternate power-off of the high-capacity memory 130B and thehigh-speed memory 130A depending upon the operation mode may reduce thepower consumption of the whole system.

During the high-speed operation mode, the high-speed memory cachecontroller 270A may provide a command, an address, and data to thehigh-speed memory 130A through the first input/output bus 620 and thehigh-speed memory 130A may provide requested data to the high-speedmemory cache controller 270A through the first input/output bus 620.

During the high-capacity operation mode, the high-capacity memory cachecontroller 270B may provide a command, an address and data to thehigh-capacity memory 130B through the second input/output bus 630 andthe high-capacity memory 130B may provide requested data to thehigh-capacity memory cache controller 270B through the secondinput/output bus 630.

Also, during the high-speed operation mode, the second memory 150 mayprovide data of a high probability of cache hit to the high-capacitymemory 130B through the second input/output bus 630 under the control ofthe second memory controller 311 and the high-capacity memory cachecontroller 270B.

Referring to FIG. 6, the processor 170 may provide a command, an addressand data to the second memory controller 311 and the second memory 150working as the system memory 151B through the input/output bus 610, andthe second memory controller 311 may provide requested data of thesecond memory 150 to the processor 170 through the input/output bus 610.

As described above, for example, when the processor 170 attempts toaccess a specific data segment in the two-level memory sub-system 500,the two-level management unit 410 may determine whether the data segmentis cached in the high-speed memory 130A. When the data segment is notcached in the high-speed memory 130A, the two-level management unit 410may determine whether the data segment is cached in the high-capacitymemory 130B. When the data segment is cached in the high-capacity memory130B, the two-level management unit 410 may fetch the data segment fromthe high-capacity memory 130B and subsequently may write the fetcheddata segment to the high-speed memory 130A. When the data segment is notcached in the high-capacity memory 130B, the two-level management unit410 may fetch the data segment from the second memory 150 working as thesystem memory 151B and subsequently may write the fetched data segmentto the high-speed memory 130A and the high-capacity memory 130B. Becausethe high-speed memory 130A and the high-capacity memory 130B work as thecaches 131 and 135 for the second memory 150 working as the systemmemory 151B, the two-level management unit 410 may further execute dataprefetching or similar cache efficiency processes known in the art.

As an example of a process of the two-level memory sub-system 500 ofFIG. 6, in response to a request for a data operand, it may bedetermined whether the data operand is cached in the high-speed memory130A working as the memory caches 131 and 135. When the data operand iscached in the high-speed memory 130A, the operand may be returned fromthe high-speed memory 130A to a requestor of the data operand throughthe first input/output bus 620 in the high-speed operation mode.

Also, during the high-speed operation mode, the second memory 150 mayprovide the data of the high probability of cache hit to thehigh-capacity memory 130B through the second input/output bus 630 underthe control of the second memory controller 311 and the high-capacitymemory cache controller 270B. The data of the high probability of cachehit provided from the second memory 150 to the high-capacity memory 130Bthrough the second input/output bus 630 during the high-speed operationmode may have a high probability of cache hit during the high-capacityoperation mode.

In accordance with an embodiment of the present invention, while thehigh-speed memory 130A provides data of a cache hit to the high-speedmemory cache controller 270A through the first input/output bus 620 as apart of the input/output bus 610 due to the cache hit of the high-speedmemory 130A in the high-speed operation mode, the second memory 150 mayalso provide data of the high probability of cache hit in thehigh-capacity operation mode to the high-capacity memory 130B throughthe second input/output bus 630 as another independent part of theinput/output bus 610.

Therefore, both the high-speed memory 130A and the high-capacity memory130B may store data having a high probability of cache hit, therebyreducing a latency caused by the access of the processor 170 to thesecond memory 150.

When the data operand is not cached in the high-speed memory 130A, itmay be determined whether the data operand is cached in thehigh-capacity memory 130B. When the data operand is cached in thehigh-capacity memory 130B working as the memory caches 131 and 135, thedata operand may be cached from the high-capacity memory 130B into thehigh-speed memory 130A and then returned to the requestor of the dataoperand through the second input/output bus 630 in the high-capacityoperation mode.

When the data operand is not cached in the high-capacity memory 130Bworking as the memory caches 131 and 135, it may be determined whetherthe data operand is stored in the second memory 150 working as thesystem memory 151B. When the data operand is cached in the second memory150, the data operand may be cached from the second memory 150 into thehigh-speed memory 130A and the high-capacity memory 130B working as thememory caches 131 and 135, respectively, and then returned to therequestor of the data operand through the input/output bus 610.

When the data operand is not stored in the second memory 150 working asthe system memory 151B, the data operand may be retrieved from the massstorage 250, cached into the second memory 150, cached into thehigh-speed memory 130A and the high-capacity memory 130B working as thememory caches 131 and 135, respectively, and then returned to therequestor of the data operand.

As described above, the bus between the handshaking interface unit andthe processor 170 may be a transactional bus including one or more ofthe PCIE bus and the DMI bus, or any other suitable transactional bus ofa small-enough transaction payload size (e.g., cache line size such as64 or 128 bytes). For example, when the second memory 150 works as thesystem memory 151B, the second memory 150 may be accessed at thegranularity of a cache line (e.g. a 64-byte or 128-Byte cache line), atwhich a memory sub-system including the first memory 130 working as theexternal memory caches 131 and 135 and the system memory 151 accesses amemory. Thus, when the second memory 150 is deployed as the systemmemory 151B within the memory sub-system, the second memory 150 may beaccessed at the same level of granularity as the first memory 130 (e.g.,the DRAM) included in the same memory sub-system. The couplingrelationship among the second memory controller 311 and the first andsecond memories 130 and 150 of FIGS. 4 and 5 may not necessarilyindicate particular physical bus or particular communication channel. Insome embodiments, a common memory bus or other type of bus may be usedto communicatively couple the second memory controller 311 to the secondmemory 150. For example, in one embodiment, the coupling relationshipbetween the second memory controller 311 and the second memory 150 ofFIGS. 4 and 5 may represent a DDR-typed bus, over which the secondmemory controller 311 communicates with the second memory 150. Thesecond memory controller 311 may also communicate with the second memory150 over a bus supporting a native transactional protocol such as thePCIE bus, the DMI bus, or any other suitable bus utilizing atransactional, protocol and a small-enough transaction payload size(e.g., cache line size such as 64 or 128 bytes).

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A memory system comprising: a first memory deviceincluding a first memory and a first memory controller suitable forcontrolling the first memory to store data; a second memory deviceincluding a second memory and a second memory controller suitable forcontrolling the second memory to store data; and a processor suitablefor executing an operating system (OS) and an application to access adata storage memory through the first and second memory devices, whereinthe first and second memories are separated from the processor, whereinthe second memory controller transfers a signal between the processorand the second memory device based on at least one of values of ahandshaking information field included in the signal, wherein the firstmemory includes a high-capacity memory, which has a lower latency thanthe second memory and operates as a cache memory for the second memory,and a high-speed memory, which has a lower latency than thehigh-capacity memory and operates as a cache memory for thehigh-capacity memory, wherein the first memory controller includes ahigh-capacity memory cache controller suitable for controlling thehigh-capacity memory to store data, and a high-speed memory cachecontroller suitable for controlling the high-speed memory to store data,wherein the second memory controller and the second memory communicatewith each other through an input/output bus, wherein the high-speedmemory cache controller and the high-speed memory communicate with eachother through a first input/output bus, which is a part of theinput/output bus, in a high-speed operation mode, wherein thehigh-capacity memory cache controller and the high-capacity memorycommunicate with each other through a second input/output bus, which isanother part of the input/output bus, in a high-capacity operation mode,and wherein the high-capacity memory caches data of the second memorythrough the second input/output bus under a control of the second memorycontroller and the high-capacity memory cache controller in thehigh-speed operation mode.
 2. The memory system of claim 1, wherein thesecond memory controller includes: a handshaking interface suitable fortransferring the signal between the second memory device and theprocessor; and a register suitable for temporarily storing data read outfrom the second memory.
 3. The memory system of claim 1, wherein the atleast one of values of the handshaking information field indicates thesignal one of a data request signal from the processor to the secondmemory, a data ready signal from the second memory to the processor anda session start signal from the processor to the second memory.
 4. Thememory system of claim 3, wherein the data request signal includes acommand and an address for the second memory device.
 5. The memorysystem of claim 3, wherein the second memory controller includes astorage unit and wherein the second memory controller reads data fromthe second memory and temporarily stores the read data in the storageunit in response to the data request signal.
 6. The memory system ofclaim 5, wherein the second memory controller provides the data readysignal to the processor when the second memory controller temporarilystores the read data in the storage unit in response to the data requestsignal.
 7. The memory system of claim 6, wherein the processor providesthe session start signal to receive the read data temporarily stored inthe storage unit in response to the data ready signal.
 8. The memorysystem of claim 1, wherein the first memory device is a volatile memorydevice.
 9. The memory system of claim 1, wherein the second memorydevice is a nonvolatile memory device.
 10. The memory system of claim 9,wherein the nonvolatile memory device is a nonvolatile random accessmemory device.
 11. A memory system comprising: a first memory deviceincluding a first memory and a first memory controller suitable forcontrolling the first memory to store data; a second memory deviceincluding a second memory and a second memory controller suitable forcontrolling the second memory to store data; and a processor suitablefor accessing the first and second memories, wherein the second memorycontroller transfers a signal between the processor and the secondmemory device based on at least one of values of a handshakinginformation field included in the signal, wherein the first memoryincludes a high-capacity memory, which has a lower latency than thesecond memory and operates as a cache memory for the second memory, anda high-speed memory, which has a lower latency than the high-capacitymemory and operates as a cache memory for the high-capacity memory,wherein the first memory controller includes a high-capacity memorycache controller suitable for controlling the high-capacity memory tostore data, and a high-speed memory cache controller suitable forcontrolling the high-speed memory to store data, wherein the secondmemory controller and the second memory communicate with each otherthrough an input/output bus, wherein the high-speed memory cachecontroller and the high-speed memory communicate with each other througha first input/output bus, which is a part of the input/output bus, in ahigh-speed operation mode, wherein the high-capacity memory cachecontroller and the high-capacity memory communicate with each otherthrough a second input/output bus, which is another part of theinput/output bus, in a high-capacity operation mode, and wherein thehigh-capacity memory caches data of the second memory through the secondinput/output bus under a control of the second memory controller and thehigh-capacity memory cache controller in the high-speed operation mode.12. The memory system of claim 11, wherein the second memory controllerincludes: handshaking interface suitable for transferring the signalbetween the second memory device and the processor; and a registersuitable for temporarily storing data read out from the second memory.13. The memory system of claim 11, wherein the at least one of values ofthe handshaking information field indicates the signal as one of a datarequest signal from the processor to the second memory, a data readysignal from the second memory to the processor and a session startsignal from the processor to the second memory.
 14. The memory system ofclaim 13, wherein the data request signal includes a command and anaddress for the second memory device.
 15. The memory system of claim 13,wherein the second memory controller includes a storage unit, andwherein the second memory controller reads data from the second memoryand temporarily stores the read data in the storage unit in response tothe data request signal.
 16. The memory system of claim 15, wherein thesecond memory controller provides the data ready signal to the processorwhen the second memory controller temporarily stores the read data inthe storage unit in response to the data request signal.
 17. The memorysystem of claim 16, wherein the processor provides the session startsignal to receive the read data temporarily stored in the storage unitin response to the data ready signal.
 18. The memory system of claim 11,wherein the first memory device is a volatile memory device.
 19. Thememory system of claim 11, wherein the second memory device is anonvolatile memory device.
 20. The memory system of claim 19, whereinthe nonvolatile memory device is a nonvolatile random access memorydevice.